1. Field of the Invention
The present invention relates to an internal combustion engine configured to ignite a premixed air-fuel mixture introduced into a cylinder by compression self ignition. The present invention also relates to a combustion control device for such an engine.
2. Background Information
Japanese Laid-Open Patent Publication No. 7-71279 discloses an example of an internal combustion engine configured to ignite a premixed air-fuel mixture introduced into a cylinder by compression self ignition. An advantage of igniting a premixed air-fuel mixture by compression self ignition is that combustion reactions start substantially simultaneously in a plurality of places inside the cylinder, thus enabling a shorter combustion time and a more stable combustion performance than spark ignition to be accomplished even when the air-fuel ratio is lean. Thus, NOx emissions can be greatly reduced because the engine can be operated on a lean air-fuel mixture at a low combustion temperature. Meanwhile, a disadvantage of this combustion method is that the usable operating range, i.e., the load region and rotational speed (engine speed) region in which engine operation is possible, is very limited due to the knocking limit and the misfiring limit. More specifically, the flame propagation is different from that obtained with spark ignition in that the substantially simultaneous self ignitions of the fuel in the air-fuel mixture inside the combustion chamber causes the combustion pressure to rise more rapidly than in the case of spark ignition. Consequently, operation of the compression self ignition engine becomes more difficult as the load increases. Thus, it is difficult to employ such an engine in applications requiring a wide operating region, such as in a vehicle.
Japanese Laid-Open Patent Publication No. 2001-323829 (corresponds to U.S. Pat. No. 6,497,213) discloses an internal combustion engine configured to vary the combustion start timing inside a combustion chamber using an ozone generating device provided in an intake port. More specifically, the combustion speed is controlled by changing an angle of the combustion start timing. Japanese Laid-Open Patent Publication No. 2001-020842 discloses another internal combustion engine in which the amount of radicals produced for radical reactions is controlled by setting the discharge start timing of a capacitive discharge or an inductive discharge of a spark plug or by providing two types of spark plugs having different capacitive discharge characteristics. By controlling the radical production amount, the combustion stability is controlled.